Vortex forming apparatus and method

ABSTRACT

Apparatus for separating, or classifying according to size, particles being carried by a primary flow of gas is disclosed. A vortex is imposed on the primary flow by a vortex forming device in the form of a central annular hub member having an outer periphery, angularly spaced nozzles leading to the periphery and an interior conduit for supplying high pressure gas to the nozzles. The nozzles are dimensioned and positioned to discharge a ring of high velocity jets of gas perpendicular to the primary flow and generally tangent to the periphery to create a vortex to submit the particles to centrifugal force, facilitating the separation or classification thereof.

United States Patent [191 Schaller 14 1 May 27, 1975 VORTEX FORMING APPARATUS AND METHOD [75] Inventor: Robin E. Schaller, Burnsville, Minn.

[73] Assignee: Donaldson Company, Inc.,

Minneapolis, Minn.

[63] Continuation-impart of Ser. No. 261,602, June 12,

1972, abandoned,

[52] U.S. Cl. 55/261; 55/432; 55/447;

137/812; 137/604; 209/144; 239/D1G. 7 [51] Int. Cl. 804C 3/00 [58] Field 01' Search 137/808-812,

137/604; 239/298, 399, 403, 405, 463, DIG. 7; 209/133, 144, 211; 55/261, 447, 468, 471,

3,351,080 11/1967 Datwyler et a1. 137/812 X 3,402,815 9/1968 Jarreby 209/144 X 3,466,853 9/1969 Buick et a1 209/144 X 3,483,973 10/1969 lager 209/139 R 3,620,370 11/1971 Swayze 209/144 3,744,220 7/1973 Klein 55/261 Primary Examiner-Frank W. Lutter Assistant ExaminerRa1ph .1. Hill Attorney, Agent, or FirmMerchant, Gould, Smith & Edell [5 7] ABSTRACT Apparatus for separating, or classifying according to size, particles being carried by a primary flow of gas is disclosed. A vortex is imposed on the primary flow by a vortex forming device in the form of a central annular hub member having an outer periphery, angularly spaced nozzles leading to the periphery and an interior conduit for supplying high pressure gas to the nozzles. The nozzles are dimensioned and positioned to dis charge a ring of high velocity jets of gas perpendicular to the primary flow and generally tangent to the periphery to create a vortex to submit the particles to centrifugal force, facilitating the separation or classification thereof.

8 Claims, 10 Drawing Figures [56] References Cited UNITED STATES PATENTS 1,464,113 8/1923 Ryding .1 55/261 X 2,125,086 7/1938 Rourke 209/144 2,496,281 2/1950 Fisher 55/468 X 3,042,202 7/1962 Work 209/144 3,060,664 10/1962 Morawski 55/460 X PATENTEUWN I975 3885931 SHEET JET INLET FLOW SCAVENGE now 8% 1 8% INLET rgow I ounzr FgOw QC VORTEX FORMING APPARATUS AND METHOD CROSS REFERENCE'TO RELATED APPLICATION This is a continuation in part of my copending patent application Ser. No. 261,602, filed June l2, l972, now abandoned.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates generally to devices for form ing a vortex in a moving flow of fluid, and more particularly relates to apparatus for imposing a vortex on a primary flow of particle carrying gas to centrifugally separate the particles from the gas.

DESCRIPTION OF THE PRIOR ART:

it is generally recognized that particles can be separated from a gas stream by subjecting the particles to the centrifugal force field present in a vortex flow. In the prior art, the necessary vortex flow was usually es tablished by inlet vanes, powered rotors or gas inlet ducts tangent to the outer periphery of a separation chamber. in such devices, the outwardly directed centrifugal force is opposed by a particle drag force acting radially inwardly. This drag force adversely affects particle collection efficiency because it tends to prevent some particles from reaching the outer periphery of the separation chamber where scavenging normally occurs. It occurred to me, after studying the prior art systems and the forces being developed on the particles to be separated, that collection efficiency could be enhanced if a vortex flow pattern were to be established having a radially outwardly directed velocity component in at least a portion of the primary flow. I further conceived that such a vortex flow pattern could be imposed by means of a ring of tangentially directed jets positioned generally perpendicular to the primary flow. I found that excellent separation efficiencies could be obtained by utilizing high velocity, low flow jets. To further en hance the vortex forming ability of the jets, I provide a suitably curved surface adjacent to each of the jets to enhance a Coanda effect (that is, attachment of the jets to the peripheral surface of the vortex forming mem ber).

A search of the prior art revealed that attempts had been made in the past to create a vortex by means of a ring of tangentially directed nozzles. The Ryding U.S. Pat. No. l,464,l 13, issued Aug. 7, 1923, discloses a gas cleaner having a tangential dirty gas inlet to establish an initial whirling motion of the incoming gas, followed by four rings of tangentially directed nozzles to accelerate the rotation of the dirty gas. it is evident from reviewing the Ryding disclosure that Ryding needed the tangential inlet to fully establish the vortex. Rydings nozzle design is of the type normally used with a low pressure supply of air, and the small ventilation blower of Ryding is not capable of producing jets traveling at sonic velocity. If Ryding did not have the tangential inlet, so that the rings of nozzles had to provide all of the swirl, at least thirty to fifty percent of the outlet flow would have to be provided by the nozzles in order to obtain a proper vortex. It is not feasible with vehicle air cleaners, for example, to provide such a large volume of clean air to the nozzles. Despite his use of rings of nozzles, Rydings device is still basically a low pressure, high flow vortex former.

An air classifier in which a ring of nozzles is used is disclosed in the .laeger US. Pat. No. 3,483,973, issued Dec. 16, 1969. The Jaeger classifier is also a low pressure, high flow device, because I00 percent of the flow passes through the nozzles. .laeger does not have a primary axial flow of gas that carries the particles to be separated. ln .laeger, all of the gas flow is through the nozzles, and the material to be separated is dropped from above into the air stream created by the nozzles. The .laeger device is basically a plurality of tangantial inlets in the form of tangentially directed low velocity nozzles. It does not differ in kind from other prior art vortex formers. In general, none of the prior art of which I am aware is capable of establishing a significant radially outwardly directed velocity component to aid in overcoming the particle drag force.

SUMMARY OF THE INVENTION In the present invention, the vortex is generated by a ring of tangentially directed, sonic or near sonicvelocity jets discharging in a direction generally perpendicular to the primary gas flow. .let velocities of approximately 1,000 feet per second can easily be obtained by supplying compressed air at 10-15 psi to a suitable nozzle or orifice from a compressor. If desired, a suitable nozzle configuration can be used so as to obtain supersonic jet velocity.

To enhance the swirl created by the jets, a curved ramp surface is provided adjacent the outlet of each jet to enhance attachment of the jet to the surface. This is referred to as the Coanda effect in fluid mechanics. This Coanda effect tightens the ring of jets to improve the vortex forming action of the jets. One advantage of the present invention over prior art structures such as those shown in Jaeger or Ryding is that llIlSqVOl'ICX forming device will create a suitable vortex by itself, without need for tangential inlets or the like. Another significant advantage is that the present invention introduces only about 8-10 percent of the total outlet air in forming the jets. This small amount of air can easily be supplied by a standard compressor. Tests have shown that with only 8 percent nozzle flow, an air cleaner incorporating the present invention will separate out virtually l00 percent of all particles above 10 microns in size, and 50 percent of particles 5 microns in size. The radially outwardly directed velocity component established by the ring of high velocity jets tends to throw the dirt particles outwardly against the walls of the separation chamber, at which point they can be easily separated from the remaining axial flow of clean gas. In its broadest sense, the present invention is a vortex former for an axial gas flow. It can be used in devices such as air cleaners, including reverse flow air cleaners, particle classifiers, as a diverter in a pneumatic powder conveying tube and as a scrubber to clean fine particles from the air. In the last case, a liquid such as water would be sprayed in combination with the air nozzles to both scrub and centrifugally separate dirt particles from the air passing through the vortex. Other applications for the present invention may occur to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWlNGS FIG. 1 is a view in side elevation of an air cleaner incorporating the present invention, portions thereof being shown in section and portions being broken away;

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1;

FIGv 3 is an enlarged fragmentary sectional view of the vortex forming device taken along line 33 of FIG.

FIG. 4 is a view in perspective of the vortex forming device shown in FIG. 3;

FIG. 5 is a graph showing the effect of an increasing jet pressure ratio on air cleaning efficiency given a constant flow relationship;

FIG. 6 is a graph comparing efficiency of the present invention with that of a typical low pressure, high flow rate prior art device;

FIG. 7 shows schematically a typical relationship among inlet flow, jet inlet flow, scavenge flow and out let flow in an air cleaner constructed according to FIG, 1;

FIG. 8 is a schematic elevational view, partly in section, of a particle classifying device constructed in accordance with my invention;

FIG. 9 is an enlarged sectional view taken along line 9-9 of FIG, 8', and

FIG. 10 is a vertical sectional view of a reverse flow air cleaner utilizing the vortex former of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The air cleaner of FIG. 1 has a cylindrical housing 10 having an open dirty air inlet end 11 and an axially located clean air discharge tube 12. Tube l2 is a cylindrical tube extending through an end wall 13 of the air cleaner. The inlet end of the tube 12 extends well into the interior of the separating chamber 14 defined by the housing 10. When the air cleaner is used with an internal combustion engine, the outer end (not shown) of the tube 12 is connected to the carburetor intake to provide clean air for combustion. Tube 12 is of smaller diameter than housing 10 to provide an annular scav enging chamber 14a between it and the cylindrical side wall. A scavenging opening I6 is formed in the cylindrical side wall of housing 10 adjacent end wall 13. A tangentially extending discharge tube 17 is connected to opening 16. Mounted on the bottom end of tube 17 is an automatic dust evacuator valve 18. Valve 18 is normally constructed from aflexible material such as rubber and is provided with a slot or discharge opening at its bottom end. Fluctuations in pressure within the system cause the valve to periodically open to discharge dirt being removed from the air stream by the air cleaner.

A vortex forming member 20 is mounted in chamber 14 between inlet 11 and outlet 12. Vortex forming member 20 includes a closed cylindrical casing 21 and an annular hub member 22. The outside diameter of annular hub member 22 is generally equal to the outside diameter of casing 21. Casing 21 comprises an end cap 210 connected by means of screws or the like to one face of hub member 22, a tubular center portion 21b connected at one end to the other face of hub member 22, and a dome-shaped inlet end cap 21c connected to the free end of center portion 21b. The cylindrical vortex forming member 20 is fixedly mounted coaxially with respect to cylindrical housing l0 by means of a pair of conduit members 24 and 25 each connected at opposite ends to casing 21 and the side wall of housing 10. Conduit member 25 extends through the side wall of housing 10 and is in communication with the interior of casing 21. The free end of conduit member 25 is adapted to be connected to a compressor 27 which then supplies air or other gas under pressure to the interior of closed cylindrical casing 21.

The construction of annular hub member 22 is shown in more detail in FIGS. 3 and 4. Hub member 22 is an annular. ring-like metal member having an outer, generally circular peripheral surface 28, and an inner, generally circular peripheral surface 29 formed coaxially with surface 28. The inner and outer peripheral surfaces 28 and 29 are connected by parallel, spaced faces 30, 30a. The downstream face 30 is generally plain while the upstream face 30a is shown in FIG. 4. Thus, the hub member 22 has a central opening which is in communication with the interior of casing 21.

The inner peripheral surface 29 and the face 300 define a circular edge 31. Formed in this edge 31 are a plurality of evenly spaced pockets or slots 32 which extend radially outwardly through a predetermined portion of the hub member from inner peripheral surface 29. As viewed from FIG. 3, the pockets 32 are somewhat L-shaped, the short leg of the L being remote from surface 29 and extending generally in the direction of the desired jet formation. The outer peripheral surface 28 is formed to include a like plurality of generally radially disposed walls 33 which are located and extend inwardly a sufficient distance to overlap the corresponding outwardly extending pockets 32. In the preferred embodiment, the walls 33 are spaced a short distance from the closed ends of the previously described L-shaped pockets in the direction of the jet formation. A hole or orifice 34 is drilled or otherwise formed through each wall 33 into the outermost end of the corresponding generally L-shaped pocket 32. The orifices I 34 extend generally tangentially with respect to the outer periphery 28 and perferably, all of the orifices 34 lie in the same plane so that when air pressure is provided to the interior of hub member 22, a ring of generally tangentially directed jets of air is provided generally perpendicular to the flow of air through the air cleaner. Each orifice 34 is preferably of much smaller diameter than the cross sectional dimension of the pocket 32, and the thickness of the wall is preferably less than the diameter of the orifice. Under these circumstances the maximum velocity of the jet at the vena contracta, occurs at or outside the orifice, when it maximizes the energy interchange between the jet and the axial air flow. Extending in a downstream direction from the innermost edge of each wall 33 is a ramp surface 35 that is curved in a radially increasing manner to enhance the attachment of the adjacent jet to the surface of the hub member. The orifices 34 are preferably tangent to the surfaces 35, or substantially so. As shown schematically in FIG. 3, the resultant jet of air 36 tends to follow the curved ramp surface 35 to thereby tighten the vortex being created by the ring of jets.

During operation of the air cleaner of FIGS. 1-4, the engine draws air through the inlet 11 to comprise a primary flow over the axially located vortex forming member 20. Because no inlets fins or tangential inlet is provided, the air initially flows axially through the housing 10. Compressor 27 is operated to provide approximately 10-l5 psi of air pressure to the interior of casing 21 to supply air under pressure to the pockets 32 and the nozzle openings 34. The air escaping through the nozzles 34 forms a ring of high velocity, tangentially directed jets that impose a vortex on the axial primary flow of air. The jets are directed generally perpendicular to the primary flow of low pressure air so that the previously described radially outwardly directed velocity component is obtained. In FIG. 3, this radially out wardly directed velocity component is schematically designated Vr while the main jet velocity vector is designated Vj and the tangential velocity component is designated Vt. The ring of high velocity jets immediately imposes a vortex on the axial flow causing the dirt particles to be thrown outwardly against the cylindrical side wall from where they are finally discharged through the scavenging outlet opening 16. The clean air from which the dirt particles have been removed exits through the clean air discharge tube 12. FIG. 7 schematically compares the various flows through the system. With a dirty air inlet flow of I00 percent and a clean air outlet flow (Q0) of I00 percent, a jet inlet flow (Qj) of 8 percent is sufficient to achieve high air cleaning efficiency. In this case, the scavenging flow of 8 percent is equal to the jet inlet flow. Tests were conducted with a jet flow rate (Qj) of 8-!0 percent of the clean air flow (Q0). It was found that for optimum efficiency, a jet pressure ratio Pl/PZ (see FIG. 3) of ap proximately two should be maintained across the ori fices 34, where the gas geing cleaned is air, thereby providing sonic gas velocity at the jet throat. As shown in FIG. 5, an increase of the jet pressure ratio PI/PZ beyond this value at a constant value of the jet flow ratio Qj/Qo does not significantly increase collection effi ciency. AC fine standard test dust collection etficiencies of 80 percent with Qj/Qo at 8 percent; and 85 percent with Qj/Qo at 10 percent, have been obtained. For particles 10 microns and larger, collection efficiency approached 100 percent. In FIG. 6, the efficiency of the present invention is compared with typical prior art systems. Assuming a constant jet pressure ratio Pl/PZ, FIG. 6 shows that maximum efficiency is obtained with these jets at a relatively low jet flow ratio Qj/Qo. The dashed line of FIG. 6 is an estimate of air cleaning effi ciency of a typical low pressure, high flow prior art device such as that shown in the Ryding patent or in the Oehlrich et al. US. Pat. No. 3,l99,268, issued Aug. 10, 1965, again assuming a constant jet pressure ratio. In these prior art devices, it can be seen that a drastic increase in secondary or jet flow might be provided in order to obtain high efficiency. In most applications, it is simply not practical to provide such a large secondary flow.

The air cleaner of FIG. 1 could be changed in a number of ways without departing from the invention. If desired, the dirty air inlet 11 could be made tangent to the cylindrical housing to provide an initial swirl. Although this is not normally necessary, it may be desirable in some unusual design situations. In like manner, inlet vanes could also be used to impart an initial swirl to the air. Particle removal is not restricted to the scavenge outlet shown in FIG. I. Any suitable known means of removing separated particles from the air stream could be used. Other means for supporting the vortex forming member could also be used. An aerodynamicallyshaped radial support would be one alternative to the conduit member shown. Some of the advantages of this air cleaner over prior art air cleaners include increased efficiency, particularly for particles 5 microns and larger, selfscavenging (no separate scavenge blower is required), automatic bypassing by shutting offjet flow Qj when air cleaning is unnecessary (no separate bypass ducting is required) and ease of preventing icing in the system.

It should be noted that for any given air cleaner of the type shown in FIG. 1, there exists an optimum value for the axial distance L between the vortex forming member 20 and the inlet to tube 12. This optimum is dependent upon the particular hub-nozzle configuration that is used, and will have to be determined empirically for any given air cleaner design. The optimum value corre sponds to a maximum collection efficiency.

FIG. 8 shows a particle classifier having a construction similar to the air cleaner of FIG. 1. This classifier has a cylindrical housing with a vaned inlet 41 formed in the cylindrical side wall adjacent one end thereof to impart an initial swirl to the air entering the housing. The vortex forming member 42 is constructed like the vortex forming member of FIG. 1, except that it is bolted at one end to an end plate 43 of the housing. Again, the vortex forming member 42 and the cylindrical chamber are coaxially aligned. The air which carries the particles to be classified thus enters the vaned inlet 41, which imparts an initial swirl to the air. The particle carrying air then passes through the vortex formed by the ring ofjets on the vortex forming member 42, as previously described. A blower 45 is provided to create a suction through the axially located fine fraction outlet 46. The fine fraction is collected by a suitable filter means 47 and deposited in a container 48. The coarse fraction is drawn off through a tangen tial outlet 49 by means ofa suction created by a blower 50. The coarse fraction is also separated by a suitable filter S1 and deposited in a container 52. The cut size can be adjusted by changing the various air flows and pressures in a manner well known to those skilled in the art.

FIG. 10 discloses a reverse air cleaner in which the air carrying the particles to be removed enters the open top end of a cylindrical housing and passes downwardly through an annular chamber formed between the housing and the axially positioned vortex forming member 56. Vortex forming member 56 is mounted on a high pressure air supply conduit 57 as described for the system of FIG. 1. In this case, however, the clean gas outlet means is an axial outlet conduit 58 which extends coaxially through the center of vortex forming member 56. The bottom end of outlet conduit 58 extends into the separation chamber below the fing of nozzles, and the upper end of conduit 58 extends upwardly from the top end of the housing. Attached to the bottom end of cylindrical housing 55 is a truncated cone-shaped portion 550 having attached to the open bottom end thereof a particle collection chamber housing 59. In this case, the air passes downwardly through the vortex forming ring of jets and then reverses flow to pass upwardly through the axial outlet conduit 58. During passage through the vortex, the heavier parti cles are thrown outwardly by centrifugal force against the walls of the housing where gravity exerts a force causing the particles to fall downwardly into collection chamber 59. Once again, the cut size or efficiency is determined by the various air flows and forces within the system.

What is claimed is:

1. Apparatus for inertially separating particles from a fluid medium, comprising:

a. a chamber having an inlet for a flow of particle carrying fluid, and an outlet;

b. a ring like vortex forming member mounted in said chamber between said inlet and outlet, having a central opening and an outer periphery, passage means including a plurality of spaced nozzles leading from said central opening to said outer periph cry, and conduit means connected to said central opening for supplying high pressure fluid to said nozzles;

c. said nozzles being positioned in said vortex forming member to discharge a ring of high velocity jets of fluid outwardly into said chamber generally per pendicular to the flow of the particle carrying fluid and generally tangentially with respect to said periphery to impose a vortex flow on the flow of particle carrying fluid and submit the particles to centrifugal force, permitting separation thereof;

d. each of said nozzles comprising a straight. orifice of length no greater than its diameter configured to restrict the flow of fluid and generate a jet of substantially sonic velocity when supplied with fluid at a suitable pressure; and

e. said periphery of said vortex forming member hav ing curved ramp surface means formed adjacent each nozzle to enhance the Coanda effect on each said jet to enhance attachment of the jet to said peh riphery of the vortex forming member.

2. The apparatus of claim 1 wherein said outer peripheral surface of said hub member includes a plurality of generally radially extending walls each having one of said tangentially directed nozzles therein, and a like plurality of said ramp surfaces therebetween, each of said ramp surfaces being curved in a radially increasing manner downstream from said nozzle to enhance said Coanda effect.

3. The apparatus of claim 1 wherein a plurality of slots extend from said central opening into said vortex forming member, wherein a like plurality of walls are formed in said outer periphery which extend inwardly a sufficient distance to overlap corresponding slots, and wherein a nozzle is formed between each said slot and corresponding wall.

4. Vortex forming apparatus, comprising:

a. a generally cylindrical housing having gas inlet means and gas outlet means defining a low pressure. high volume gas flow path therebetween;

b. vortex forming means mounted in said gas flow path, said vortex forming means including an annu lar hub member with a central opening, and with an outer peripheral surface mounted coaxially with said cylindrical housing, said hub member having passage means, including a plurality of tangentially directed nozzles, extending between said central opening and said outer peripheral surface, each of said nozzles being a straight orifice of length no greater than its diameter, so as to be capable of generating a jet having its greatest velocity component downstream from the orifice;

c. means for supplying a relatively low volume flow of high pressure gas through said nozzles to discharge jets of gas into said chamber at substantially sonic velocity to impose a vortex flow on the low pressure gas flow; and

d. said outer peripheral surface being formed to have a plurality of curved ramp surfaces formed adjacent said plurality of nozzles to provide a Coanda effect on the jets to enhance attachment of the jets to the peripheral surface of the hub member.

5. A method of imposing a vortex flow on a low pressure, relatively high volume primary flow of a gas. said gas carrying particles to be separated therefrom, comprising the steps of discharging through a ring of flow restricting orifices, having lengths no greater than their diameters, a relatively low volume secondary flow of gas, positioning the orifices to provide generally tangentially directed, spaced jets, directing the jets generally perpendicular to said primary flow, and maintaining said jets at a velocity generally equal to or greater than sonic velocity to submit the particles to centrifugal force in the vortex formed thereby.

6. The method of claim 5 including the step of dis charging each of said jets against a Coanda surface to tighten the ring of jets to improve the vortex forming action thereof.

7. The method of claim 6 including the step of maintaining the secondary flow volume at approximately 8-10 percent of the primary flow volume.

8. A particle separator employing vortex forming ap paratus. comprising:

a. a generally cylindrical housing mounted with a longitudinal axis extending generally vertically and having gas inlet means and gas outlet means defining a low pressure, high volume primary gas flow path therebetween, said gas inlet means being an inlet for particle carrying gas located at a top end of said housing;

b. vortex forming means mounted in said primary gas flow path, said vortex forming means including a single ring of generally tangentially directed nozzles positioned to discharge a secondary flow of gas generally perpendicular to said primary flow, and said gas outlet means being an axial outlet conduit for clean gas extending from said top end of said housing and through said ring of nozzles into said housing;

c. means for supplying a relatively low volume secondary flow of high pressure gas through said nozzles to discharge jets of gas into said chamber at substantially sonic velocity to impose a vortex flow on the low pressure primary gas flow; and

d. a particle collecting chamber mounted at a lower end of said housing, said vortex subjecting particles carried by said gas through said inlet to centrifugal forces to permit gravitational separation thereof from the clean gas being carried out of said housing through said axial outlet conduit. 

1. Apparatus for inertially separating particles from a fluid medium, comprising: a. a chamber having an inlet for a flow of particle carrying fluid, and an outlet; b. a ring-like vortex forming member mounted in said chamber between said inlet and outlet, having a central opening and an outer periphery, passage means including a plurality of spaced nozzles leading from said central opening to said outer periphery, and conduit means connected to said central opening for supplying high pressure fluid to said nozzles; c. said nozzles being positioned in said vortex forming member to discharge a ring of high velocity jets of fluid outwardly into said chamber generally perpendicular to the flow of the particle carrying fluid and generally tangentially with respect to said periphery to impose a vortex flow on the flow of particle carrying fluid and submit the particles to centrifugal force, permitting separation thereof; d. each of said nozzles comprising a straight, orifice of length no greater than its diameter configured to restrict the flow of fluid and generate a jet of substantially sonic velocity when supplied with fluid at a suitable pressure; and e. said periphery of said vortex forming member having curved ramp surface means formed adjacent each nozzle to enhance the Coanda effect on each said jet to enhance attachment of the jet to said periphery of the vortex forming member.
 2. The apparatus of claim 1 wherein said outer peripheral surface of said hub member includes a plurality of generally radially extending walls each having one of said tangentially directed nozzles therein, and a like pluralIty of said ramp surfaces therebetween, each of said ramp surfaces being curved in a radially increasing manner downstream from said nozzle to enhance said Coanda effect.
 3. The apparatus of claim 1 wherein a plurality of slots extend from said central opening into said vortex forming member, wherein a like plurality of walls are formed in said outer periphery which extend inwardly a sufficient distance to overlap corresponding slots, and wherein a nozzle is formed between each said slot and corresponding wall.
 4. Vortex forming apparatus, comprising: a. a generally cylindrical housing having gas inlet means and gas outlet means defining a low pressure, high volume gas flow path therebetween; b. vortex forming means mounted in said gas flow path, said vortex forming means including an annular hub member with a central opening, and with an outer peripheral surface mounted coaxially with said cylindrical housing, said hub member having passage means, including a plurality of tangentially directed nozzles, extending between said central opening and said outer peripheral surface, each of said nozzles being a straight orifice of length no greater than its diameter, so as to be capable of generating a jet having its greatest velocity component downstream from the orifice; c. means for supplying a relatively low volume flow of high pressure gas through said nozzles to discharge jets of gas into said chamber at substantially sonic velocity to impose a vortex flow on the low pressure gas flow; and d. said outer peripheral surface being formed to have a plurality of curved ramp surfaces formed adjacent said plurality of nozzles to provide a Coanda effect on the jets to enhance attachment of the jets to the peripheral surface of the hub member.
 5. A method of imposing a vortex flow on a low pressure, relatively high volume primary flow of a gas, said gas carrying particles to be separated therefrom, comprising the steps of discharging through a ring of flow restricting orifices, having lengths no greater than their diameters, a relatively low volume secondary flow of gas, positioning the orifices to provide generally tangentially directed, spaced jets, directing the jets generally perpendicular to said primary flow, and maintaining said jets at a velocity generally equal to or greater than sonic velocity to submit the particles to centrifugal force in the vortex formed thereby.
 6. The method of claim 5 including the step of discharging each of said jets against a Coanda surface to tighten the ring of jets to improve the vortex forming action thereof.
 7. The method of claim 6 including the step of maintaining the secondary flow volume at approximately 8-10 percent of the primary flow volume.
 8. A particle separator employing vortex forming apparatus, comprising: a. a generally cylindrical housing mounted with a longitudinal axis extending generally vertically and having gas inlet means and gas outlet means defining a low pressure, high volume primary gas flow path therebetween, said gas inlet means being an inlet for particle carrying gas located at a top end of said housing; b. vortex forming means mounted in said primary gas flow path, said vortex forming means including a single ring of generally tangentially directed nozzles positioned to discharge a secondary flow of gas generally perpendicular to said primary flow, and said gas outlet means being an axial outlet conduit for clean gas extending from said top end of said housing and through said ring of nozzles into said housing; c. means for supplying a relatively low volume secondary flow of high pressure gas through said nozzles to discharge jets of gas into said chamber at substantially sonic velocity to impose a vortex flow on the low pressure primary gas flow; and d. a particle collecting chamber mounted at a lower end of said housing, said vortex subjecting particles carried by said gas through said inlet to centrifugal forces to permit gravitational separation thereof from the clean gas being carried out of said housing through said axial outlet conduit. 